The invention relates to thermal barrier coatings (TBCs). More particularly, the invention relates to TBCs applied to superalloy gas turbine engine components.
The application of TBCs, such as yttria-stabilized zirconia (YSZ) to external surfaces of air-cooled components, such as air-cooled turbine and combustor components is a well developed field. U.S. Pat. No. 4,405,659 to Strangman describes one such application. In Strangman, a thin, uniform metallic bonding layer, e.g., between about 1-10 mils, is provided onto the exterior surface of a metal component, such as a turbine blade fabricated from a superalloy. The bonding layer may be a MCrAlY alloy (where M identifies one or more of Fe, Ni, and Co), intermetallic aluminide, or other suitable material. A relatively thinner layer of alumina, on the order of about 0.01-0.1 mil (0.25-2.5 μm), is formed by oxidation on the bonding layer. Alternatively, the alumina layer may be formed directly on the alloy without utilizing a bond coat. The TBC is then applied to the alumina layer by vapor deposition or other suitable process in the form of individual columnar segments, each of which is firmly bonded to the alumina layer of the component, but not to one another. The underlying metal and the ceramic TBC typically have different coefficients of thermal expansion. Accordingly, the gaps between the columnar segments enable thermal expansion of the underlying metal without damaging the TBC.
U.S. Pat. No. 6,060,177 to Bornstein et al. (the disclosure of which is incorporated by reference herein as if set forth at length) describes use of an overcoat of chromia and alumina atop a yttria-stabilized zirconia (YSZ) TBC. Such an overcoat may protect against sulfidation attack and oxidation and may significantly extend the operational life of the component.